Devices & Valves / Animations / Control valve sizing
Control Valve Sizing & Cavitation
Size a control valve and watch what the fluid does inside it. Pick the fluid, valve style and operating cases: the valve opens to the calculated travel and shows clean flow, cavitation, flashing or choked gas flow.
What you are watching
The valve opens to the travel needed for the case you pick (Min, Normal or Max). Fluid speeds up through the narrowest gap, the vena contracta, where pressure is lowest. Downstream, part of that pressure recovers. The graph under the valve shows this pressure profile to scale against the liquid's vapour pressure.
- Clean flow: the lowest pressure stays above the vapour pressure.
- Cavitation: the pressure dips below the vapour pressure, bubbles form, then collapse (red flashes) as pressure recovers. Expect noise and trim erosion.
- Flashing: the outlet pressure is below the vapour pressure, so the bubbles never collapse and the outlet flow is two-phase.
- Choked gas: flow reaches sonic speed at the vena contracta; shock cells appear and extra ΔP adds no flow.
Things to try
- Load Cooling water, then switch the valve style from globe to butterfly. The lower FL recovers more pressure, so the vena contracta dips deeper.
- Load Hot water (cavitating) and lower P1 or raise P2 until the cavitation warning clears.
- Load Natural gas (choked) and raise P2 until the flow is no longer choked.
Theory and formulas: Control valves: Cv sizing.